This application is based on and claims the priority under 35 U.S.C. xc2xa7119 of German Patent Application 100 11 238.2, filed on Mar. 8, 2000, the entire disclosure of which is incorporated herein by reference.
The invention relates to a high capacity air conditioning system especially for a passenger transport aircraft, having a redundant and multi-staged admixing of recirculation air to serve several air conditioning zones within the aircraft while achieving a redundant fault tolerant protection against air duct icing in the air conditioning system during operation.
It is generally known to provide air conditioning for the passenger cabin spaces in commercial passenger transport aircraft. To achieve this, highly compressed engine bleed air is tapped from the engines, and supplied into one or more air conditioning units (e.g. so-called air conditioning packs) where the high pressure air is expanded to an appropriate pressure for introduction into the pressurized aircraft fuselage and also cooled to a lower temperature. Then, the cooled air with an appropriate pressure is delivered through an air duct network into the several passenger cabin air conditioning zones.
Due to the large thermal load of the passengers, lighting, etc. within the passenger cabin air conditioning zones, it is generally necessary to cool the incoming supplied air very considerably in the air conditioning units to provide a sufficient cooling capacity. Thereby, the air temperature very often reaches low temperatures far below the freezing point of water. The external atmospheric fresh air, which is supplied into the air conditioning system in the form of engine bleed air, has a sufficiently high moisture content, especially at lower altitudes in the atmosphere, so that this moisture will be condensed out of the air and frozen in the form of ice, frost or snow, when the air is cooled to very cold temperatures in the air conditioning unit.
For this reason, it is necessary to protect the air duct system connected downstream of the respective air conditioning unit from the danger of such snow, frost and ice accumulating therein, i.e. by preventing the ice-forming conditions. This is important, because accumulations of such ice can ultimately lead to a partial or complete blockage of the air flow through the affected air duct, or can lead to the formation of loose snow and ice deposits that travel through the duct system with the air flowing therethrough until they reach a warmer location and melt, thereby causing uncontrollable liquid puddling and/or leaks. In either case, and especially if a duct blockage reduces or totally stops the supply of fresh air to the passenger cabin air conditioning zones, this will lead to discomfort or health risks for the passengers.
For the above reasons, it is generally required in the field to avoid the danger of ice formation in air conditioning systems in aircraft, by various conceptual solutions to this problem. Various conventionally known solutions will now be discussed. A first conceptual starting point for a solution is to avoid reducing the temperature of the air conditioning air below the freezing point of water, i.e. to maintain the output air of the air conditioning packs above the freezing point. In that case, however, in order to provide the required total cooling capacity or energy, it is necessary to provide a correspondingly increased mass flow of the cooled air, which requires tapping more energy-rich bleed air from the engines.
Moreover, in order to achieve comfortable passenger air conditioning zone inlet temperatures and sufficiently high ventilation properties, it is typical to mix recirculated passenger cabin air (or generally fuselage interior air) with a corresponding quantity of the cooled air. Such a measure is generally known as xe2x80x9cair recirculationxe2x80x9d. Such a solution is realized in the Airbus A300-600 and A310 aircraft, which, however, consumes an undesirably high quantity of engine bleed air for cooling the passenger air conditioning zones, since the degree of cooling of the air in the air conditioning units is limited to remain above the freezing point. Thus, the fuel consumption of the engines is undesirably increased.
The Boeing B747 aircraft also realizes or embodies the above discussed solution. In that aircraft, however, the air conditioning system attempts to raise the temperature of the cooling air by mixing cold air and recirculation air in a first recirculation stage following a distribution manifold. This measure is carried out with reference to each air conditioning zone. The desired ventilation and comfort properties are achieved by addition of further recirculation air in a second stage. Splitting or separating the recirculation into stages in this manner allows a reduction of the air duct cross-sectional sizes and the associated weight of the air ducts between the stages. However, the danger of ice accumulation with the associated danger of air duct blockage is not completely avoided, because the first air mixing stage is only provided downstream of the air distribution stage. This is true for both normal operation as well as failure or fault mode operation.
In the event of a failure of one recirculation unit, the supply of air provided by the still-operable second recirculation unit cannot compensate for this failure of the other recirculation unit. Moreover, in this context, the air conditioning unit outlet temperature remains limited to above the freezing point, just as in the above discussed Airbus A300-600 and A310 aircraft. In order to compensate for the loss of cooling energy, the only solution is to increase the mass flow of energy-rich engine bleed air, which leads to an increased fuel consumption.
Efforts have been made to avoid this above mentioned disadvantage in the Airbus A340 and A320 aircraft, in that the entire cooling and recirculation air is collected or pooled together in a common mixing unit. In that manner, a high failure redundancy is achieved to allow a fault tolerant or emergency operation in the event of cooling air and/or recirculation air supply failures.
In view of the above described conceptual basis, it is apparent that icing of the cooling air supply ducts within the pressurized fuselage will not be avoided, and complicated duct arrangements connecting to a mixing chamber as well as additional flow-influencing components will be necessary, in order to optimally configure and embody the mixing process. Moreover, a residual danger of icing in areas of air flow separation still remains and cannot be completely excluded. A further disadvantage is that rather large dimensions of the air duct cross-sections and a rather high weight of the air duct system are unavoidable, in comparison to the arrangement of the Boeing B747, in order to provide the entire required air conditioning air quantity for achieving the required ventilation and comfort characteristics, in a central mixing chamber and then to supply and distribute this air from the common central mixing chamber to the several separate zones.
As a further development, the Boeing B777 aircraft to some extent realizes a combination of the systems of the Boeing B747 and the Airbus A340/A320 aircraft. Namely, the Boeing B777 aircraft uses a central mixing chamber, to which are allocated a first cool air supply with a constant recirculation air admixing and a second cool air supply without a constant recirculation air admixing. To complete the air flow in order to achieve corresponding required ventilation and comfort properties, a further constant recirculation air quantity is locally admixed into the previously mixed air described above. However, the second cool air supply without the recirculation air admixing suffers the above mentioned disadvantages of duct icing, or more directly the resultant temperature limitations of the air and the associated cooling energy losses that are necessary for avoiding duct icing. Also in the event of a failure of the recirculation air admixing into the cooling air supply, both cooling air supply air temperatures must be maintained above the water freezing point, which thus represents a considerable loss of cooling energy capacity.
Furthermore, U.S. Pat. No. 4,517,813 discloses an air conditioning system with a so-called air-mix water-separator manifold, into which appropriately tempered air quantities are supplied from two sides, whereby this supplied air is formed from partial air quantities of warm recirculated cabin air, that has been combined with cool air quantities provided from an air cycle machine (ACM). The outlets of this branching or distribution manifold are respectively connected to various fuselage areas of a passenger aircraft, for example the cockpit or flight deck, a forward passenger cabin or zone, and an aft passenger cabin or zone. This manifold serves to mix the air quantities that are supplied into the manifold, while simultaneously separating any water or moisture out of the humid supply air, presumably also in order to avoid ice formation or ice accumulation or the transformation of air moisture into snow crystals. However, it appears that such a protection against icing in the air ducts only relates to a single stage (and not a multi-stage) recirculation air admixture, and does not provide any protection against blockages of downstream connected air ducts of multiply branched ducts or air lines of a complex high capacity air conditioning system. Particularly, no suggestions and no motivations toward the present invention of the present application would have been provided by the above-mentioned U.S. Patent.
In view of the above it is an object of the invention to provide an air conditioning system for a passenger aircraft, which achieves a fault or failure tolerant redundant operation and protection against icing of air ducts through a trimmed admixture of recirculation air in connection with the operation of a high capacity cooling system, with a minimum cold air mass flow consumption and maximum total air quantity provision for the passenger cabin air conditioning zones, in order to achieve the prescribed ventilation and passenger comfort properties. The invention further aims to avoid or overcome the disadvantages of the prior art, and to achieve additional advantages, as apparent from the present specification.
The above objects have been achieved according to the invention in an air conditioning system with a redundant multi-stage admixture of recirculation air for a passenger aircraft, in which several air conditioning zones are to be respectively air conditioned. Throughout this specification, the term xe2x80x9czonexe2x80x9d or xe2x80x9cair conditioning zonexe2x80x9d refers to a respective temperature zone or respective passenger cabin zone (e.g. a first class cabin zone versus an economy cabin zone), which are to be separately or individually supplied with air conditioning air, for example having different temperatures, different air flow rates, different ratios of fresh air relative to recirculation air, or the like. Further, throughout this specification, the term xe2x80x9cair linexe2x80x9d and the term xe2x80x9cair ductxe2x80x9d are each intended to cover any type of passage for conveying an air flow therethrough, including ducts, pipes, hoses, air shafts, channels, and the like.
The air conditioning system according to the invention comprises at least two air conditioning units which respectively receive high pressure air that contains water vapor, and which respectively expand and cool the supplied air to temperatures below the freezing point of water. The air conditioning system further includes at least two pre-mixing units arranged in the aircraft fuselage, whereby a respective individual pre-mixing unit is directly individually connected downstream to each one of the air conditioning units. A respective recirculation unit is further connected to each pre-mixing unit to supply a recirculation air flow of recirculated cabin air (or generally fuselage interior air) into the pre-mixing unit. The quantity of recirculation air provided by each respective recirculation unit to the connected pre-mixing unit is variable and controllable to provide the proper mixture of recirculation air and air conditioning pack air.
The at least two pre-mixing units are each connected in turn at the outlet or downstream side to an air distribution mixing chamber, which collects and pools together the various flows of pre-mixed air that it receives, and provides respective portions of this collected pre-mixed air to several respective air distribution outlets that are respectively in turn each individually connected to an after-mixing or post-mixing unit, to which further a recirculation supply line from at least one local recirculation unit is connected. Thereby, an additional variable controllable quantity of recirculation air drawn from fuselage interior spaces is respectively admixed into the air provided from the air distribution mixing chamber, separately for each respective air conditioning zone, i.e. with a separately adjustable or variable admixing ratio for each individual air conditioning zone. The respective total mixed air provided by each post-mixing unit respectively, is then supplied into the respective associated air conditioning zone.
With the above arrangement, the present inventive air conditioning system allows the air conditioning pack air provided at the outlets of the air conditioning units to be cooled to low temperatures below the freezing point of water, so as to achieve a high cooling capacity, while nonetheless preventing the formation of ice, frost or snow in the air conditioning system, due to the admixing of recirculated fuselage interior air in respective pre-mixing units connected directly downstream to the outlets of the air conditioning units. The pre-mixing units preferably mix just enough recirculation air into the incoming cold air to raise the temperature above the freezing point of water, e.g. to achieve a temperature not more than 1xc2x0 C. above the freezing point. Thereby, icing conditions are reliably prevented throughout the air conditioning system, while maintaining the highest possible cooling capacity, smallest possible duct and mixing chamber sizes upstream of the final post-mixing units, and the highest degree of flexibility or adjustment leeway for the final post-mixing units to achieve the final air characteristics (e.g. temperature, flow rate, fresh air proportion, etc.) required for a particular air conditioning zone.
Furthermore, the present inventive air conditioning system provides redundancy and thus fault tolerant operation, by collecting and pooling together the pre-mixed air flow from at least two pre-mixing units respectively cooperating with at least two air conditioning units, while distributing the pre-mixed air as required to all of the separate air conditioning zones. Thus, in the event of the failure of one of the pre-mixing units and/or one of the air conditioning units, the remaining available air conditioning unit and pre-mixing unit will still provide pre-mixed air into the air distribution mixing chamber, which in turn will distribute the air to all of the necessary individual air conditioning zones. In such a fault tolerant or emergency operating mode, the maximum available total cooling capacity will be less than the maximum full capacity achieved when all air conditioning units are operating, but cooling air will still be supplied to all necessary air conditioning zones, and the outlet temperature of one or more remaining operational air conditioning units can be reduced to help compensate for the loss of maximum air conditioning capacity, while still avoiding the dangers of ice formation.
As another advantage, the present inventive air conditioning system achieves an individualized control and mixing ratio of recirculated fuselage interior air for each respective individual air conditioning zone, despite the common pooling of the supply air in the air distribution mixing chamber. This is achieved by the post-mixing units that are individually interposed in the separate air supply lines connecting the air distribution mixing chamber to the several respective air conditioning zones. Thus, different zones can have different ratios of recirculated air, different temperatures, or the like.
All of the above advantages are achieved together and simultaneously in a unified air conditioning system according to the invention.